1. Field of the Invention
The present invention generally relates to a method and apparatus for removing impurities from coal, and more particularly to a method and apparatus for separating ash and pyritic sulfur from coal and for achieving high levels of ash and pyritic sulfur rejection without fine grinding.
2. Description of the Related Art
Froth flotation, a surface-based process, has been widely employed in the coal industry for upgrading fine coal. Since coal is naturally hydrophobic, it can be separated from most types of mineral matter based on differences in surface wettability. Compared to other fine particle processing techniques, froth flotation offers many advantages including simple circuit layout, high unit capacity, good selectivity, and low equipment and maintenance costs.
Generally, coal is made into an aqueous slurry and then subjected to a physical beneficiation process such as froth flotation to remove impurities. However, while froth flotation is very selective for removing well-liberated mineral matter, it is less effective if the feed coal contains a large amount of composite particles (i.e., so-called "middlings") containing, for example, pyrite. This difficulty arises due to flotation recovery being a function of both particle size and particle composition. The optimum particle size for flotation is usually 100.times.200 mesh (Rastogi and Aplan, 1985. "Coal Flotation as a Rate Process," Minerals and Metallurgical Processing, Vol. 2, pp. 137-147). Therefore, a middlings particle of the optimum particle size can be recovered more readily than well-liberated coal particles that are outside the optimum range. The separation is further complicated when pyrite becomes hydrophobic due to superficial oxidation (Yoon et al., "On the Hydrophobicity of Coal Pyrite," Proceedings, 4th International Conference on Processing and Utilization of High Sulfur Coals, Idaho Falls, Id., August, 1991, pp. 241-253).
Recent studies indicate that surface-based processes such as froth flotation are inefficient in removing pyrite (e.g., a material having a relatively high specific gravity) from fine coal. This shortcoming is due to pyrite becoming hydrophobic under certain conditions and to the inability of flotation to effectively reject middlings particles.
The shortcomings of flotation with regard to pyrite rejection may be overcome by using a density-based separation technique which utilizes centrifugal force to increase the gravitational pull acting on the particles. Several such centrifugal separators, which are capable of treating flotation-size coal, have recently been introduced to the minerals processing environment. These include the Mozley Multi-Gravity Separator (Tucker, et al., "Modelling of the Multi-Gravity Separator," Les Techniques, December 1992, pp. 45-49), the Falcon Concentrator (Lins, et al., "Performance of a new Centrifuge (Falcon) in Concentrating a Gold Ore from Texada Island, B.C., Canada," Minerals Engineering, Vol. 5, 1992, pp. 1113-1121) and the Knelson Concentrator (Knelson, "The Knelson Concentrator: Metamorphosis from Crude Beginning to Sophisticated Worldwide Acceptance," Minerals Engineering, Vol. 5, 1992, pp. 1091-1097). These known separators are believed to be particularly useful for removing pyrite from coal because of the large differences in specific gravity.
However, ash-forming minerals (e.g., SiO.sub.2, Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, etc.) are removed less efficiently by gravity separators because of their lower specific gravity. These separators are also incapable of handling ultrafine clay "slimes" that report with the clean coal by entrainment.
Thus, the inefficient rejection of both ash and pyritic sulfur is a common problem in industrial fine coal cleaning circuits and has hitherto not been addressed by the conventional systems.